DiffEditor: Introduce a base class for the different views

... of the diff. Currently that is side-by-side and unified, just
as before.

Change-Id: I62a5462344c4b4ae652899f9d5b2936aa5a692b8
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>

src/plugins/diffeditor/diffeditor.qbs

@@ -34,6 +34,8 @@ QtcPlugin {
         "differ.h",
         "diffutils.cpp",
         "diffutils.h",
+        "diffview.cpp",
+        "diffview.h",
         "selectabletexteditorwidget.cpp",
         "selectabletexteditorwidget.h",
         "sidebysidediffeditorwidget.cpp",